The present invention pertains to a data transmission system. More particularly, the present invention pertains to a data transmission network allowing communication between a plurality of data stations with each data station being permitted access within a reasonable time interval to the data network for transmission of data, while avoiding interference which would result from simultaneous data transmission from more than one station.
Data transmission networks consist of a number of data stations interconnected for data exchange. Such data stations might include computer systems, small scale computers, and various peripheral equipment such as intelligent or non-intelligent terminals. Each data station is coupled to the network by a communication unit. The data network can be any communication medium including, by way of examples, twisted pair wire, coaxial cable, or fibre optical cable. The stations of the network might be connected in a star, a ring, or a bus configuration, among others. A bus structure is usually preferred because it is easier to install and to cable.
In a data bus, however, control of the various data stations is desirable to avoid data transmission by more than one station at any one time, since such simultaneous transmission would result in interference or collision of the various data signals. Different techniques are utilized to avoid such interference, including frequency division multiplexing and time division multiplexing. Time division multiplexing is often preferred because of its comparative cost advantages. Time division multiplexing can be achieved by a central control system or by a distributed control system. In a distributed control system, a central control system can be imitated by defining temporarily one of the communication units as the master unit with temporary control of the network. Alternatively, each communication unit can have equal status and can utilize the network whenever it has data for transmission and finds the network communication bus unoccupied. However, it is desirable that the time for obtaining network control be comparatively small with respect to the time for data transfer.